Cucumbers turn yellow if they have been on the vine too long (over-mature)

https://www.gardeningknowhow.com/edible/vegetables/cucumber/when-to-pick-a-cucumber-how-to-prevent-yellow-cucumbers.htm

yellow cucumbers are generally not fit for consumption. A yellow cucumber can also be the result of a virus, too much water, or a nutrient imbalance. In some instance, yellow cucumbers are derived from planting a yellow-fleshed cultivar, such as the lemon cucumber, which is a small, lemon-shaped, pale yellow variety.